How to Stop Route Advertisement from Old ExpressRoute Circuit?

Question

How can I stop the route advertisement from my old ExpressRoute circuit after migrating to a new circuit, especially when my service provider is unresponsive to requests for removing routes?

PS - Based on common issues that we have seen from customers and other sources, we are posting these questions to help the Azure community.

Answer 1

When migrating to a new ExpressRoute circuit, it is crucial to ensure that routes from the old circuit are no longer advertised to avoid any network conflicts or issues. In your case, the service provider has been unresponsive to requests to remove the old routes. Therefore, an alternate solution is required.

One effective method is to use AS PATH prepending. This technique involves lengthening the AS PATH attribute of the routes being advertised from the old circuit, making them less preferred compared to the new routes. However, it is important to note that this configuration change needs to be made on the service provider’s end and cannot be implemented from the Azure side.

Here are the steps to follow: 1. Request AS PATH Prepending: Contact your service provider and request them to apply AS PATH prepending on the routes being advertised from the old ExpressRoute circuit. 2. Verify Route Preferences: Ensure that the new routes from your new ExpressRoute circuit are being preferred over the old ones by monitoring the routing tables.

By using AS PATH prepending, the new routes will automatically become the preferred paths, effectively stopping the advertisement from the old circuit.

Resources:

• Azure ExpressRoute Documentation
• BGP with ExpressRoute
• AS PATH Prepending

Hope this helps. If you have any follow-up questions, please let me know. I would be happy to help.

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